1. Field of the Invention
The present invention relates generally to a mobile ad hoc network (MANET), i.e., a network using a plurality of mobile terminals/stations, and more particularly to a medium access control (MAC) protocol layer module for wireless local area networks (LANs) based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and a method for transmitting a flooding ad hoc traffic indication message (FATIM).
2. Description of the Related Art
A mobile ad hoc network (MANET) is an infrastructure network implemented without a fixed router or host and a base station. In the MANET, a connection between mobile nodes is made using multi-hopping technologies based on a peer-to-peer level. Topology of the MANET can be dynamically changed, and the MANET can carry out a self-forming function and a self-healing function. Because the MANET is not a network in which only a fixed based station supports a mobile service, nodes enable a network routing infrastructure architecture in an ad hoc form. No limitation is present when the respective nodes included in the MANET can freely move, and therefore, the MANET uses a protocol adaptable to a structural variation according to fast movement of a node.
A medium access control (MAC) protocol supporting the MANET for wireless local area networks (LANs) based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard is based on a distributed coordination function (DCF), and specifies a power save mode (PSM) for a power saving mechanism. Each node in the MANET has basic computing and wireless communicating capabilities and usually limited capacity of battery power supply. In MANET research, energy efficient techniques have been very important issues from all aspects of system and protocol design.
The DCF specifies the PSM consisting of 3 states of power consumption. The 3 states include an off state, an awake state, and a sleep state. In the off state, a wireless interface consumes no power. In the awake state, the wireless interface consumes a necessary level of power required for a transmission operation, a reception operation, or a listening operation (idle mode). However, very little power is consumed in the sleep state in which the wireless interface cannot carry out the transmission or reception operation. In the DCF, time is divided into fixed intervals, that is, beacon intervals, and each beacon interval is divided into an ad hoc traffic indication message (ATIM) window and a following data transmission window. During the ATIM window, each node wakes up and exchanges the announcements for next data transmission with neighbors. The nodes sending or receiving ATIMs stay awake for data transmission and other nodes go into the sleep state. The data packets usually move forward, hop by hop, consuming one beacon interval per hop.
As described above, nodes operating in the DCF PSM are in either the awake state or the sleep state. The nodes in the awake state can transmit or receive packets and consume different amounts of energy according to operating states. The nodes in the sleep state cannot communicate but consume very little energy.
FIG. 1 is an explanatory view illustrating the PSM in the MANET. Referring to FIG. 1, each node reduces a power supply when not transmitting or receiving a packet, and wakes up periodically, that is, at each beacon interval (BI). Then, as the node sends information indicating when its own node will transmit a packet or recognizes when its own node will receive a packet, it participates in communication, thereby reducing power consumption.
As described above, the time in the DCF is divided into beacon intervals (BIs) 10. Each BI 10 is divided into an ad hoc traffic indication message (ATIM) window 12 and a following data transmission window 14. In the beginning of each BI, all nodes wake up, exchange beacon frames and synchronize with one another using time stamps in the beacon frames. After the exchange of the beacon frames, the announcements for next traffic are transmitted. Each node listens for these announcements and determines whether to stay awake. If a node does not have any traffic to be sent or received at the end of the ATIM window, the node transitions to the sleep state until the beginning of the next BI.
For example, a node-A (1) sends an ATIM to a node-B (2) during the ATIM window 12, notifying the node-B (2) that a packet to be sent is present. Thus, the node-B (2) receives the ATIM during the ATIM window 12 and sends an acknowledgement (ACK) message to the node-A (1). Then, the node-A (1) transmits the packet to the node-B (2) and the node-B (2) sends an ACK message to the node-A (1) upon receiving the packet.
All the nodes in the sleep state periodically stay awake for a predetermined time period at each BI. In the IEEE 802.11 standard, the predetermined time period is referred to as an ATIM window. During the ATIM window, the nodes exchange beacon frames and synchronize with one another. When an ATIM frame to be sent is present, each node sends the ATIM frame. A destination of the ATIM frame is determined by a destination of its message.
When a message to be sent is destined for a specific neighbor node, the destination of an ATIM frame is the same as the destination of the message. This ATIM frame is referred to as a unicast ATIM frame. A neighbor node receiving the unicast ATIM frame responds through an ACK frame to notify that the unicast ATIM frame has been appropriately received. At the end of the ATIM window, the neighbor node stays awake and receives a message.
However, when a message to be sent is destined for all the neighbor nodes, the destination of an ATIM frame is given to all the neighbor nodes. Accordingly, this frame is referred to as a broadcast ATIM frame. The neighbor nodes receiving the frame stay awake at the end of the ATIM window, and receive broadcast messages.
An ATIM frame can be sent only during the ATIM window, and the ATIM window repeatedly and periodically starts at a start point of a beacon interval (BI). If the ATIM window expires, a node sending the ATIM frame and a node receiving the ATIM frame transmit packets in the awake state during the rest of the BI. The other nodes, i.e., the nodes not sending or receiving an ATIM frame, go into the sleep state and save power.
In order for consumption power to be saved in a standby state, the BI must be set to be relatively longer than the ATIM window. However, when a corresponding node desires to transmit a message, the node must wait until the beginning of the next BI. As a result, the transmission delay time is longer.
More specifically, when a broadcast message is sent to the entire network in a multi-hop ad hoc network based on the pre-existing IEEE 802.11 DCF PSM, a long transmission delay time is required until the message reaches all the nodes because the message is transmitted one hop during each BI. In the case of a network protocol establishing a path to a destination depending upon a network broadcast as in an application program or an ad hoc on-demand distance vector (AODV) routing algorithm frequently employing a network broadcast message, its performance can be severely degraded according to the long transmission delay time. For example, assuming that the longest path in the network is 4 hops and a BI is set to 500 msec, a minimum time period of 2 sec is required for a message to reach a destination and an additional time period of 2 sec is required for a response message to be received. This operation will be described in more detail herein below with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B are views illustrating an example of transmitting a packet according to a conventional PSM mechanism. Referring to FIG. 2A, network including node-s 100 and node-a to node-g 120-180, the longest path from the source node-s 110 is 4 hops (from node s to node-e or g), and at least 4 beacon intervals are needed for the packets to reach all nodes. If there are collisions between the nodes, more than 4 beacon intervals may be needed. Referring to FIG. 2B, packet forwarding steps are illustrated when flooding originates at the node-s 110. The arrows illustrated in FIG. 2B indicate the transmission of ATIMs during each of the BIs 30-60. For simplicity, no ACK is returned and arrows for data packet transmissions are omitted. In the conventional PSM mechanism, the problem is not just the latency, but also an amount of power consumption. Further, the flooding is more problematic. Each node in experiencing flooding cannot determine whether the ATIM is duplicated before receiving an ATIM, so each node must stay awake even for the redundant messages.
Consequently, when the pre-existing IEEE 802.11 DCF PSM mechanism is employed in the multi-hop ad hoc network, an amount of consumption power is inversely proportional to a response delay time. The more consumption power is saved, the longer a response time is.